I shall introduce the hydrodynamics that underlies the way in which microorganisms, such as bacteria and algae, and fabricated microswimmers, swim. For such tiny entities the governing equations are the Stokes equations, the zero Reynolds number limit of the Navier-Stokes equations. This implies the well-known Scallop Theorem, that swimming strokes must be non-invariant under time reversal to allow a net motion. Moreover, biological swimmers move autonomously, free from any net external force or torque. As a result the leading order term in the multipole expansion of the Stokes equations vanishes and microswimmers generically have dipolar far flow fields. I shall introduce the multipole expansion and describe physical examples where the dipolar nature of the bacterial flow field has significant consequences, the velocity statistics of a dilute bacterial suspension and tracer diffusion in a swimmer suspension.

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